feat: add pg_rsa

[barrownicholas]

What kind of change does this PR introduce?

Adds small pg_rsa extension that I made (designed to implement rsa signing and verifying in postgres function).

What is the current behavior?

Currently, there is no way to sign in RSA directly in a postgres function.

What is the new behavior?

Adds RSA-functions: see https://github.com/barrownicholas/pg_rsa#functions

Additional context

Currently, I am trying to connect to Google APIs using a service account and a self-signed JWT, using a PEM private key. Google only supports RSA256. Unfortunately for me, there are no out-of-the-box solutions for RSA-signing in Postgres... until now! See https://github.com/barrownicholas/pg_rsa#functions.

[barrownicholas]

Confirming that this passed on my end:

• nix build (nix build .#psql_17.exts.pg_rsa) succeeded
• docker image built successfully:

docker build -f Dockerfile-15 -t supabase-postgres:15 . 
docker run --rm -d --name pg_rsa_test -e POSTGRES_PASSWORD=postgres supabase-postgres:15
docker exec -it pg_rsa_test psql -U supabase_admin -d postgres -c "CREATE EXTENSION pg_rsa;"
docker exec -it pg_rsa_test psql -U supabase_admin -d postgres -c "SELECT pg_rsa_test();" # built-in test suite

 pg_rsa_test 
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 t
(1 row)

[barrownicholas]

(note to self: after pr approved, need to add to: https://github.com/supabase/supabase/blob/master/packages/shared-data/extensions.json)